1. Field of the Invention
The present invention relates to a noise-suppressing wiring-member for composing a printed wiring board and the printed wiring board. Also, the present invention relates to a noise suppressing structure and a multilayer printed wiring board.
The present invention claims priority on Japanese Patent Application No. 2006-181179, filed on Jun. 30, 2006, and Japanese Patent Application No. 2006-199286, filed on Jul. 21, 2006, the content of which is incorporated herein by reference.
2. Description of the Related Art
In recent years, a central processing unit (CPU) with a high clock frequency in a sub-microwave band (0.3 to 10 GHz), such as in a personal computer, personal digital electronics, local area wireless networks, Bluetooth, optical modules, mobile phones, game equipment, personal digital assistants, intelligent transport information systems, and the like, electronic equipment which utilizes a high-frequency bus, and an information communication device which utilizes an electric wave have been in wide-spread use in accordance with the spread of the use of the internet, and a ubiquitous society, in which a micro processing unit (MPU) will be required to have high-speed performance, multi-functions, integrated functions, and high-speed digitization performance, and in which devices will be required to offer high performance even when driven at a low-voltage, is on the horizon.
However, such a spread of these devices is becoming a controversial issue in that a radiation noise radiated from these devices and a conduction noise conducting through these devices cause malfunction of the devices, other electronic equipment, or parts thereof, and cause influences on the human body. As the noise, noise caused by impedance mismatching of conductors provided in a printed wiring board equipped with MPUs, electronic elements, and the like, noise caused by crosstalk between conductors, and noise induced by resonance between a power supply layer and a ground layer, the resonance being caused by simultaneously switching of semiconductor elements of the MPU or the like are exemplified. For example, when plural transistors in a semiconductor element mounted in a multilayer printed wiring board are simultaneously driven, unwanted high-frequency current flows in a power supply layer or a ground layer, as a result of which a potential variation occurs. The potential variation causes simultaneous switching noise in the power supply layer and the ground layer. Also, since the power supply layer and the ground layer have a parallel flat plate structure in which peripheral end parts thereof are opened, the potential variation causes resonance between the power supply layer and the ground layer, as a result of which radiation noise is radiated from the peripheral end parts.
As printed wiring boards in which occurrence of such noises are suppressed, the following printed wiring boards are known:    (1) a printed wiring board in which a metallic film made of a metal with a specific resistance greater than that of copper foil is formed on both surfaces of a power supply layer and a ground layer, the power supply layer and ground layer being formed by copper foil (Patent Document 1 (Japanese Unexamined Patent Application, First Publication No. H11-97810)); and    (2) a printed wiring board in which a film containing a conductive substance, the film having an anisotropical conductivity in the vertical direction with respect to the printed wiring board surface, is formed on both surfaces of a power supply layer and a ground layer, the power supply layer and ground layer being formed by copper foils (Patent Document 2 (Japanese Laid-Open Patent Application No. 2006-66810)).
The printed wiring board (1) is considered that a high-frequency eddy current flowing on the surface of the copper foil can be attenuated therein, as a result of which an electric potential of power supply or the like can be stabilized and the radiation of unwanted noises can be suppressed even if semiconductor elements are simultaneously switched. In order to attenuate the high-frequency current (surface current) flowing on the surface of a conductor using a metallic film with a thickness of several micro meters, the thickness being approximately equal to the surface depth, a material with a considerably high specific resistance is required depending on the frequency of the high-frequency current to be attenuated. However, it is practically impossible to acquire such a material, and thereby the printed wiring board (1) cannot exhibit sufficient noise suppressing effects.
Also, the printed wiring board (2) is considered that the high-frequency eddy current can be attenuated therein in a similar way to the above. However, a step of forming an anisotropically conductive film with a surface roughness of copper foil equal to or greater than the surface depth is complicated The printed wiring board (2) also cannot exhibit sufficient noise suppressing effects.
As a method for suppressing radiation noise, (i) a method in which an electromagnetic shielding material which reflects an electromagnetic wave is used; and (ii) a method in which an electromagnetic wave absorption material which absorbs an electromagnetic wave propagating in space is used, are exemplified. As a method for suppressing generation of a conduction noise and a radiation noise, (iii) a method in which a high-frequency current flowing in conductors is suppressed before generating a conduction noise or a radiation noise is exemplified.
However, in the method (i), although shielding effects of the radiation noise can be exhibited, the radiation noise returns to oneself by unwanted radiation or reflection or the radiation noise by the shielding material. In the case of the method (ii), since the electromagnetic wave absorption material (such as, for example, one disclosed in Patent Document 3 (Japanese Unexamined Patent Application, First Publication No. H9-93034 and one disclosed in Patent 4 (Japanese Unexamined Patent Application, First Publication No. H9-181476) is heavy, thick, and fragile, it is not suitable for a device required to be small and light. Also, the conduction noise cannot be suppressed by the methods (i) and (ii). Accordingly, the method (iii) has attracted attention.
Also, Patent Document 1 discloses that a metallic film with a high resistance is formed on each copper foil layer composing a power supply layer or a ground layer. The metallic film with a high resistance is a monolayer film or an alloy film formed by plating nickel, cobalt, tin, tungsten, and the like, which has a specific resistance higher than that of copper. The metallic film with a high resistance is considered to be able to stabilize the potential variation of a power supply layer and a ground layer and suppress unwanted electromagnetic waves (radiation noise) radiated outward by eliminating the high-frequency current, even if semiconductor elements are switched.
However, since a metal with favorable workability, such as a nickel or the like, has a small resistance, sufficient effects cannot be exhibited. In contrast, since a metal with a high resistance, such as a tungsten or the like, is very difficult to process, and therefore cannot be used for parts, such as an area surrounding a semiconductor element, in which a complicated and fine pattern is required to be formed, the metal is not practically used. Also, the suppression of radiation noise is insufficient.
Patent Document 5 (Japanese Patent Publication No. 2867985) discloses a multilayer printed wiring board in which a resistive element such as carbon, graphite, or the like, is provided between a power supply layer and a ground layer at a peripheral end part of a multilayer printed wiring board.
However, when the resistive element is provided at only the peripheral end part, only the resonance frequency is changed by varying the impedance at the peripheral end part. The electric field intensity and the magnetic field intensity at other portions of the multilayer printed wiring board are enhanced. Accordingly, the radiation noise or the like, caused by resonance, cannot be suppressed still, and further countermeasures are required.
Patent Document 6 (Japanese Patent Publication No. 2738590) discloses a capacitive printed wiring board equipped with a capacitor laminate in which a dielectric sheet is held between two conductive foil layer, the conductive foil layers being electronically connected to each device.
However, the capacitive printed wiring board is required to be thick because the capacitor laminate has a certain thickness, and therefore it is not suitable for high density mounting. When the capacitive printed wiring board is thick, the resonance easily occurs between conductors with a parallel flat plate structure, and thereby the generation of the radiation noise cannot be sufficiently suppressed.